1. Field of the Invention
The present invention relates to a method for regenerating a solid iodine filter of the type of those used in factories for reprocessing irradiated nuclear fuels in order to trap the residual iodine present in the gas effluents produced during this reprocessing.
2. Description of the Related Art
In factories for reprocessing irradiated nuclear fuels, the recovery of residual iodine contained in the gas effluents in the form of molecular iodine I2 and/or of organic compounds of iodine such as iodo-alkanes or alkyl iodides, is ensured, before discharging these gas effluents into the environment, by mineral solid traps commonly designated as <<iodine filters>>.
These are cartridges filled with porous silica or alumina beads which are impregnated with silver in nitrate form. In these iodine filters, the iodine reacts with the silver nitrate in order to form iodine compounds such as silver iodide and iodate with possibly a low presence of physisorbed molecular iodine I2.
These iodine filters form a solid waste contaminated with 129I (which is a long lifetime radioelement) which cannot be directly stored at the surface and for which no matrix for storage in depth is available today.
Consequently, in the international PCT application published under number 02/073629, a method was proposed which allows decontamination of a used iodine filter, either from iodine, or from iodine and silver, and its declassification may thereby be obtained for admission, after packaging in a cement matrix, into a surface storage center.
According to this method, the filter is decontaminated from iodine by immersion in an aqueous basic solution, which is heated to 60° C. and which contains a reducing agent such as for example ascorbic acid, able to transform the iodine compounds into soluble iodide anions (I−). It may further be decontaminated from silver, either simultaneously by adding cyanide to the aqueous basic solution used for the decontamination from iodine, or successively by washing the filter in an acid solution, typically a solution of nitric acid, capable of dissolving the silver which it contains.
If this method unquestionably represents a technical progress, it however has the drawback:                in the case when the silver is removed from the filter, of adding to the reprocessing of irradiated nuclear fuels, already generating significant volumes of non-recoverable liquid effluents, additional liquid effluents loaded with silver which will have to be treated, and        in the case when the silver is not removed from the filter, of losing the silver which remains in this filter.        
Now, it turns out that within the scope of their work on iodine filters, the Inventors noticed:                on the one hand, that against all expectations, the structure and the physical characteristics (porosity, specific surface area, . . . ) of these filters are only altered very little by use for several years in the factory, notably when they comprise an alumina support;        on the other hand, that the reducing treatment recommended in WO-A-02/073629 when it is carried out at room temperature allows very efficient decontamination of these filters from iodine without however extracting the silver present in the filters nor extensively altering the structure and the physico-chemical properties of these filters; and        finally, that it is possible after having extracted from used iodine filters both the iodine and the silver which they contain, to re-impregnate them very efficiently with silver and this by notably using as an impregnation solution, the acid aqueous solution having been used for decontaminating them from silver or a solution prepared from the latter.        
And it is on the basis of these observations that the inventors have achieved the present invention.